Currently when a response is encoded it is always encoded with the encoding the 
the server adapter. This is very useful except in the one off case where you 
need a different encoding eg. for answering a CSV document. See [1] for an 
example.

In general something like this should be enough to switch the encoding of the 
response:

aResponse contentType: ((WAMimeType main: 'text' sub: 'csv')
    charset: 'utf-16';
    yourself)

 [1] http://lists.squeakfoundation.org/pipermail/seaside/2011-November/028041.html